In recent years, in mobile communications systems represented by mobile telephone systems, the analogue modulation scheme of the first generation has gradually shifted to a digital modulation scheme of a second generation, and still, to a third generation which realizes high data transfer rates and can be used globally seamlessly.
And, for the transition periods between the generations, multi-mode or multi-band radio communications equipment which can correspond to, in addition to the respective existing radio communications systems, new radio communications systems becomes necessary. Furthermore, in such radio communications equipment, an antenna device which can correspond to a plurality of frequency bands has been required.
For example, as the antenna device which can correspond to a plurality of frequency bands, an antenna device as shown in JP-A-2001-285122 (Patent document 1) has been known. This prior-art antenna device is provided with a diplexer which sorts received signals received via the antenna into communications systems corresponding to the DCS system and GSM system different in frequencies and a four-port high-frequency switch for separating into a transmitting portion and a receiving portion of each communications system, which enables downsizing the circuit by reducing the number of components of the mobile communications device.
In addition, as another antenna device, an antenna device which, by use of a high-band high-frequency switch, simply switches power supply to a multi-band antenna having the number of bands equal to the number of corresponding frequencies of an application apparatus has also been considered.
Patent document 1
JP-A-2001-285122 (Page 3, FIG. 1)
However, the conventional antenna devices have had the following problems, and an improvement thereof has been demanded. Namely, in a case where a system using a new frequency band and also using antenna diversity is added to an existing radio communications system not using antenna diversity, a need arises for the antenna device to correspond to both conventional and new frequency bands and also correspond to the diversity. However, if the above-described correspondence is considered in the conventional schemes, this becomes a disincentive, such as preparing an antenna for each of the radio communications systems, to reduction in size and weight, which is important for mobile terminal apparatuses in the mobile communications systems.
In addition, in a case where a new radio communications system and a frequency band are added to conventional models, for example, in a case where the third-generation mobile telephone system is merged with the second-generation mobile telephone system, drastic modifications of the conventional antenna element and antenna circuit have been necessary.
The present invention has been made to solve the problems as described above, and an object thereof is to provide, by a simple construction, an antenna device which corresponds to a plurality of radio communications systems and frequency bands, such as two radio communications systems and three frequency bands, which can simultaneously carry out a reception and a transmission of different radio communications systems, and which is capable of using antenna diversity.